1. Field of the Invention
The present invention relates to a battery unit used as a power supply for, for example, an electric vehicle.
2. Description of the Related Art
An electric vehicle is equipped with a battery unit as a power supply for running. The battery unit comprises a battery module, a battery case for storing the battery module, etc. The battery case of the battery unit of this type includes a battery carrier for storing a battery, and a battery carrier cover for covering the battery carrier.
Lattice-shaped projections are formed in the bottom of the battery carrier so that a plurality of batteries are held in a fitted state. Each battery module is stored in a recess defined by the projections. Each battery module stored in the battery carrier is held between the battery carrier and the battery carrier cover. A structure of this type is disclosed in Japanese Patent No. 2758348.
At the same time, the battery needs to be charged. There are two ways of charging; one of which is a quick charge for charging in a short period of time, and the other uses, for example, a domestic power source, which takes more time for charging than the quick charge.
The battery module generates heat when the battery is charged. In particular, one great challenge for the electric vehicle is how to increase the mileage. Therefore, the largest possible battery unit (including a plurality of battery modules) tends to be mounted. Thus, in the case of the quick charge which completes charging in a short period of time, the calorific value of the battery tends to be high because the battery is charged using a high-current power supply. An increased calorific value of the battery leads to deterioration of the battery and is therefore not preferable.
The battery case is provided with a fan as a structure for cooling off the battery module while the electric vehicle is running. While the electric vehicle is running, the fan is driven to bring air into the battery case. Each battery is cooled off by the air brought in. A structure of this type is disclosed in Japanese Patent No. 2758348.
In order to improve the rigidity of the battery case, the lattice-shaped projections formed in the bottom of the battery carrier may be increased in height. However, if the projections are increased in height, the area of each battery module covered with the projections is increased. Accordingly, the area of each battery module in contact with the air brought into the battery case decreases. A decreased area of each battery module in contact with the air is not preferable in terms of the cooling of the battery module.